To prevent the ullage, that is to say the space between the fluid level in a tank and the fixed top of the tank from filling with volatile fumes, it is common practice in the oil refinery industry to provide a floating roof or pontoon on top of the fluid. As the floating roof rises or sinks with the addition or withdrawal of fluid from the tank, there is no ullage which can be filled with volatile gases. However, to prevent the escape of volatile fumes from between the tank and the floating roof, there is a need to provide a tank roof seal about the periphery of the floating roof and in contact with the side of the tank.
Heretofore a number of devices have been used to provide a sealing relationship between the floating roof of the tank and the side walls thereof. Such devices are disclosed in U.S. Pat. No. 1,650,340, issued to Glass on Nov. 22, 1927, and U.S. Pat. No. 1,735,461 issued to Haupt on Nov. 12, 1929. Both systems utilize flexible annular tubes located about the periphery of the floating roof to form a seal between the floating roof and the sides of the tank. However, it has been found that the flexible annular tube, which is generally comprised of an elastomeric material is abraded against the tank wall as the floating tank roof rises or sinks owing to the change in the amount of fluid in the tank. Such abrading reduces the effectiveness of the seal and eventually can allow vapors and sometimes liquid to escape from the tank through the seal. As stricter environment protection standards are imposed, such seals become increasingly unsatisfactory. Thus, there is a need to provide a tank seal which will meet the newest environment protection standards.
Additionally, as there are a number of tanks which are fitted with the aforementioned flexible annular seals, which must be retrofitted with tank seals which meet the newest environment protection standards, there is a need to provide a tank seal which can be used to quickly and easily retrofit roofs. It is to be understood that such retrofitting can most easily and conveniently be accomplished in situ. That is to say the seal should be retrofitable without the necessity of draining the fluid or oil from the tank. Additionally, it is noted that to retrofit in situ, neither welding nor any other process which creates a flame or sparks can be used as such procedures would create the possibility of an explosion, should highly volatile and explosive fuels be contained in the tank.
Examples of other floating seals wherein a metallic shoe is substituted for the flexible annular tube and a resilient mechanism or a weight biased mechanism is used to extend the shoe from the floating roof into contact with the side of the tank can be found in for example the following patents: U.S. Pat. No. 1,900,904 issued to Berger on Mar. 14, 1933; U.S. Pat. No. 2,587,508 issued to Moyer on Feb. 26, 1952; U.S. Pat. No. 2,772,805 issued to Grundig on Dec. 4, 1956; and U.S. Pat. No. 3,019,935 issued to Anderson on Feb. 6, 1962. These systems essentially show seals which can be included as original equipment with a floating roof. However, none of these seals are adaptable for retrofitting an existing liquid filled tank.